The referenced application Ser. No. 214,481, filed Dec. 9, 1980, Klaus Muller et al, the disclosure of which is hereby incorporated by reference, describes a sensor positioned in a sparkplug-like housing which is capable of being secured through the cylinder wall of an internal combustion engine. The sensor can sense the pressure conditions which occur in the combustion chamber upon ignition of the air-fuel mixture therein.
Combining such a sensor operational with a sparkplug has the advantage of being space-saving, and of being readily connected. This, then, requires an electrical conduction line in addition to the optical transmission or optical guide, so that ignition energy can be supplied to the unit, to form a combined sparkplug-sensor element. In a system which has been proposed, a metallic sleeve is provided within which a glass rod, or glass fiber element is located. Typically, quartz glass may be used.
The operation of sparkplugs by themselves is well known; sparkplugs, in operation, have different characteristics which are matched to the operating characteristics of the internal combustion engine, and the use to which the engine is put--for example whether it is used to drive an automotive vehicle of the passenger car type, a motor cycle, a truck, or if the combustion engine is used for small applications, such as chain saw or a lawn mower. Different characteristics are demanded of the sparkplug, thus, depending on operating loading, use, compression, speed, cooling available, setting of the carburetor or fuel injection system and the like. Consequently, the construction of sparkplugs must be matched to the engine, and it is known that, even with any type of engine, sparkplugs with different heat capacity or heat characteristics can be used. Using, for example, a sparkplug of identical construction in an engine in which the sparkplug would become highly heated, the sparkplug may become too hot and the fuel-air mixture could be ignited by elements of the sparkplug which start to glow, resulting in misfires, or glow firing, which is undesirable. If, on the other hand, the same sparkplug is installed in a different engine in which it is operated at a relatively low average temperature, the tip of the sparkplug insulator would be rapidly carbonized due to residues from combustion, and, if carbonization becomes excessive, the ignition energy may become shunted through the carbon deposits resulting in failure of ignition, and again misfires. Consequently, matching of the sparkplug to a particular engine, and its operating conditions, is important, since the sparkplug in a specific motor should become neither too hot, nor remain too cold. It is, therefor, known to provide sparkplugs with different thermal loading or heat characteristics. These different thermal characteristics, which can be referred to as "heat loading" of the sparkplug, are values which are associated with each sparkplug and based on the design thereof, so that interchangeable sparkplugs can be constructed to match any particular type of engine in the light of its operating characteristics.
When combining an optical sensor with a sparkplug, it has been found that the heat capacity thereof, or the heat characteristic, is such that the sparkplug, in operation, may overheat at the operating tip. The heat capacity of the sparkplug is low, since the glass rod in the center thereof is a poor heat conductor. The sparkplug tip, thus, will heat excessively by not being able to radiate the resulting heat. If the sparkplug is installed in an internal combustion engine which is operated at high power rating, the sparkplug may become excessively hot at the tip and glow-ignition may result, which, in effect, means that the engine will be subject to misfires.